In subjects with chronic balance disorders, approaches to medical treatment based on drugs and surgery can be effective in stabilizing the pathological processes that cause the disorders. In such subjects, these approaches can sometimes stabilize but seldom fully resolve the underlying pathological manifestations of the balance problem. Once the underlying pathological processes are medically stable, however, rehabilitation exercises frequently prove effective in reducing many of the disabling symptoms and functional problems associated with chronic balance disorders. Hence, effective treatment of chronic balance disorders typically employs combinations of medical and rehabilitation exercise treatments.
In selecting medical treatments most likely to stabilize underlying pathological processes, clinicians first determine the location, nature, and extent of the underlying pathological process. To make pathological determinations, clinicians typically rely on the results of the subject history and physical examination to develop diagnostic hypotheses, and then use site-of-lesion laboratory tests to confirm or rule out their hypotheses. In designing effective rehabilitation exercise programs, in contrast, clinicians require additional knowledge of the subject's functional impairments and adaptive response capabilities. For this reason, objective tests that isolate and quantify the functional impairments associated with balance disorders complement the information provided by site-of-lesion tests and complete the clinical information necessary for effective treatment planning and outcome documentation.
To develop methods and devices for isolating and quantifying functional impairments of the balance system, it is first necessary to understand the functional organization of the balance system. The balance system includes a number of processes that may be grouped into distinct but interdependent systems—one responsible for gaze stabilization and the other responsible for postural stabilization. The gaze stabilization system maintains the gaze direction of the eyes relative to surrounding visual targets as the subject or targets moves within the subject's environment. Information useful in managing patients with chronic balance disorders may be provided by assessing their ability to maintain dynamic visual acuity. Dynamic visual acuity refers to a subject's ability to accurately perceive a visual object while either the subject moves and the visual object is fixed, the visual object moves and the subject is fixed, or the subject and the visual object are moving independently. When the subject and/or the visual object are moving, subjects whose gaze control system functions normally can maintain their visual acuity by continuously moving their eyes to stabilize their direction of gaze relative to the visual object. Stabilizing the direction of gaze while a person moves maintains their visual acuity during activities involving active head and body movements. When individuals with impaired gaze stabilization participate in activities involving self-motion and moving objects in the surrounds, moving objects may appear blurred while stationary objects may become blurry and sometimes appear to be in motion.
Individuals with normal visual function achieve their best visual acuity when the viewed visual object is focused on a position fixed on the fovea, a small region located at the approximate center of the retina of the eye. The fovea contains the highest density or concentration of visual or cone receptors. Acuity is substantially reduced when the position of an object is displaced more than a few degrees of visual angle from the center of the fovea, and/or when the object is moving faster than two degrees per second. However, the time it takes the brain to perceive an object is short, and these precise position and velocity requirements need to be maintained for no longer than approximately 25 milliseconds for accurate perception to occur. In subjects with a history of brain disorders, two examples being stroke and traumatic brain injury, the minimum time for accurate perception to occur may be substantially increased to 100 milliseconds or more.
A detailed discussion of gaze stabilization may be found in “Disorders of the Vestibular System” edited by Robert W. Baloh and G. Michael Halmagyi and published by Oxford University Press, New York, in 1996 (Chapter 3 How Does the Vestibulo-ocular Reflex Work?, and Chapter 6, How Does the Visual System Interact with the Vestibulo-ocular Reflex?), both chapters of which are hereby incorporated herein by reference. In summary, gaze is stabilized on visual objects under a wide variety of subject and object movement combinations encountered in daily life through the cooperative interactions of five movement control systems: (1) the vestibulo-ocular reflex (VOR) system; (2) the smooth pursuit eye movement system; (3) the saccadic movement system; (4) the optokinetic movement system; and (5) the vergence eye movement system. Because the gaze stabilization system is highly adaptive, which one or combination of control systems is used varies depending on the specific conditions of the subject and visual object motion.
A number of research studies have examined the dynamic visual acuity of subjects exposed to visual objects while the subject and/or the visual target were moving. These studies were primarily concentrated on the ability of the VOR system to stabilize a subject's gaze when the subject moves and the visual object is fixed. For example, a series of studies by Demer and colleagues quantified the ability of subjects to accurately perceive fixed visual targets in the presence of active and passive head rotations. However, because isolating and quantifying the functional performance of the individual eye movement control systems were not goals in these studies, they did not disclose methods for displaying and moving visual objects that can achieve reliable isolation of the performance of individual eye movement control systems.
Aznar-Casanova et al. (2005) examined the effects of moving a relatively large visual area at different fixed velocities on the minimum spatial frequencies that could be perceived by their subjects. However, their subjects were exposed to predictably moving visual fields for periods as long as 14 seconds. Because predictable visual object motions were used, their methods could not be used to reliably isolate (predictive) saccadic eye movements from the smooth pursuit and optokinetic movements. Haarmeier and Thieer (1999) examined the visual acuity of normal subjects and patients viewing visual objects smoothly moving over a range of velocities. However, the direction of object movements was always to the right and the acuity test always administered 250 milliseconds following the onset of movement. Because several features of visual object motions were predictable, subjects may have been able to substitute predictive saccadic movements.